CtrlY

Ctrl-Y in sup will scroll the screen up one line. But Ctrl-Y may be gobbled up by your shell and interpreted as a "delayed suspend" signal, returning control to the underlying shell. If you have accidentally pressed Ctrl-Y and find yourself at a shell prompt:

 fg

...and you will be returned to your sup session.

To prevent this behavior altogether so that you may use Ctrl-Y for scrolling in sup, add the following line to your .bashrc, .zsh, .profile or equivalent shell startup script:

 stty dsusp undef